1. Field of the Invention
This invention relates in general to two cycle internal combustion engines, and more particularly to systems and devices for scavenging combustion gases in, and for lubricating, such engines.
2. Background Art
The Otto cycle for reciprocating, internal combustion engines comprises five main events: power, exhaust, intake, compression and ignition. In a four-stroke-cycle engine ("four cycle engine"), these are distinct events, which are associated with the strokes of the piston(s) within the engine cylinder(s), as follows: (1) an intake stroke, wherein an air-fuel mixture enters an upper, combustion chamber portion of the cylinder(s); (2) a compression stroke, wherein the air-fuel mixture is compressed; (3) ignition; (4) the power stroke; and (5) an exhaust stroke; that is, it requires two crankshaft revolutions in a four cycle engine to complete one Otto cycle. Lubrication oil is supplied to the piston(s), cylinder(s), crankshaft and other moving engine parts from the crankcase, either by an oil pump or by an oil splash system that splashes crankcase oil up into the cylinder(s). Each piston is fitted with piston rings to substantially prevent contamination of the oil in the crankcase by combustion products ("blowby gases") created in the combustion chamber. Each cylinder of a four cycle engine is commonly equipped with spring loaded intake and exhaust valves located in the engine head and actuated through coupling elements by a cam shaft driven by the crankshaft.
The two-stroke-cycle engine ("two cycle engine") is so called because it requires only two full strokes (one complete crankshaft revolution) to accomplish the five main events of the Otto cycle. Unlike a four cycle engine, in a two cycle engine the five main events of the Otto cycle are not distinct. The first one is an out stroke; but this stroke is a power stroke (started by ignition) and it also accomplishes exhaust and part of intake. A second (in) stroke completes the cycle by finishing intake and accomplishing compression. As compression proceeds, ignition occurs and the following out stroke starts the next cycle. Neither intake nor exhaust require separate strokes, as they are accomplished during the power and compression strokes. Unlike in a four cycle engine, however, no intake or exhaust valve is required for these events to occur. Instead, each cylinder combustion chamber is provided with an inlet port A, for entry of fresh air-fuel mixture, and an outlet port B for discharge of burnt gases, and it is the reciprocating movement of the piston C itself that opens and closes these ports as the piston skirt cyclically and repeatedly covers and uncovers them; see FIGS. 1A-1E. Fresh air-fuel mixture, mixed with lubricating oil, is drawn directly into the crankcase E through a third port (intake port) D during the compression stroke. After ignition, the air-fuel/oil mixture in the crankcase is compressed by the descending piston during the power stroke. The descending piston pumps the air-fuel/oil mixture through the inlet port into the combustion chamber as first the exhaust port and later the inlet port are uncovered by the descending piston, thereby scavenging burnt gases from the combustion chamber while simultaneously lubricating the piston and upper cylinder.
Although this method of lubricating a two cycle engine has the virtue of simplicity compared to four cycle engines, it yields a relatively high level of air pollutants from the products of combustion, a relatively low engine life, and inadequate lubrication of crankshaft bearings (unless lubricating oil is pumped directly to the bearings, as in some Detroit Diesel service engines). Prior to my invention, however, there was no satisfactory way to achieve low pollutant levels and long engine life by using sump lubricating oil within the crankcase of a two cycle engine to lubricate the upper cylinder(s), combustion chamber(s) and piston(s), and without the lubricating oil interfering with combustion of the air-fuel mixture. E. A. Buckman et al., U.S. Pat. No. 1,672,869, disclosed a two cycle engine embodying a duplex piston having upper and lower portions of different external diameters, corresponding to the internal diameters of the sleeve valve, disposed for reciprocal motion within the sleeve valve. Air-fuel mixture was admitted to the cylinder through an intake port that communicated with the lower, larger diameter portion of the sleeve valve. An annular chamber for storage of air-fuel mixture was provided above and in communication with the larger bore of the sleeve valve. Peripheral channels on the upper, smaller portion of the sleeve valve afforded communication between the storage chamber and the larger portion of the valve, such that a charge of air-fuel mixture admitted to the larger bore could be transferred to the storage chamber and compressed during the upward movement of the piston. Providing suitable reciprocating linkages between the sleeve valve and the crankshaft, however, significantly added to the complexity and weight of the engine. A two cycle engine disclosed by C. L. Stokes, U.S. Pat. No. 1,802,585, embodying a cylinder having concentric upper and lower bores of different diameter, a duplex sleeve valve reciprocal therein and having cylindrical portions corresponding in diameter to the bores in the cylinder, and a piston (albeit, a non-duplex piston) reciprocal within an upper portion of the sleeve valve, likewise suffered from the same limitation: the necessity for additional linkages between the crankshaft and the sleeve valve to reciprocate the sleeve valve. So too, the two cycle engine disclosed by W. L. Schmitz, U.S. Pat. No. 2,127,758, employing a piston within a piston arrangement, one fitted to move axially within the other, whereby a combustion chamber is formed between the heads of the two pistons. Each of these approaches lost somewhat of the advantages of two cycle engines: i.e., their relative simplicity and low weight.